If I'm correct, practice balls are also known as pressureless balls, which aren't air filled and considerably last longer. They aren't USTA approved. Regular balls are filled with air and are USTA approved and regulated for matches.

'Practice balls' are generally balls that came of the line with some defect and thus are sold at a lower price than regular balls. The extent of the defect can be a slight as the name not being printed straight and as severe as a split seam. Pressureless balls are something different altogether.

Practice balls are the same as pressureless balls (wilson calls them "practice" and gamma calls them "pressureless") so the material is harder in an attempt to make the bounce the same. They do suck
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I honestly didn't know how bad practice balls were until the other day. I tried some of those Penn Tribute pressured practice balls that someone bought for me just for rallying and they felt very hard, didn't come off the racquet correctly, didn't bounce much, and I found myself shanking more of them than the other balls for some reason.

I put them into the hopper for serving practice and yesterday I found them just as bad for serving, they just didn't feel right.